Zirconium-based cross-linking composition for use with high pH polymer solutions

ABSTRACT

A cross-linking composition comprising (a) an aqueous liquid; (b) a pH buffer; (c) a cross-linkable organic polymer; and (d) a solution of a zirconium cross-linking agent comprising the product of contacting a zirconium complex with an alkanolamine and ethylene glycol wherein the mole ratio of alkanolamine to zirconium is 2:1 to 4:1 and the mole ratio of ethylene glycol to zirconium is 1:1 to 10:1. Optionally, water, hydroxyalkylated ethylenediamine, or both are added to the zirconium complex. The cross-linking composition of this invention is useful in oil field applications, for example, for hydraulically fracturing a subterranean formation and for plugging permeable zones or leaks in a subterranean formation.

FIELD OF THE INVENTION

The present invention relates to zirconium chelate cross-linking agentsand their use in oil field applications such as hydraulic fracturing andplugging of permeable zones.

BACKGROUND OF THE INVENTION

The production of oil and natural gas from an underground well(subterranean formation) can be stimulated by a technique calledhydraulic fracturing, in which a viscous fluid composition (fracturingfluid) containing a suspended proppant (e.g., sand, bauxite) isintroduced into an oil or gas well via a conduit, such as tubing orcasing, at a flow rate and a pressure which create, reopen and/or extenda fracture into the oil- or gas-containing formation. The proppant iscarried into the fracture by the fluid composition and prevents closureof the formation after pressure is released. Leak-off of the fluidcomposition into the formation is limited by the fluid viscosity of thecomposition. Fluid viscosity also permits suspension of the proppant inthe composition during the fracturing operation. Cross-linking agents,such as borates, titanates or zirconates, are usually incorporated intothe fluid composition to control viscosity.

Typically, less than one third of available oil is extracted from a wellafter it has been fractured before production rates decrease to a pointat which recovery becomes uneconomical. Enhanced recovery of oil fromsuch subterranean formations frequently involves attempting to displacethe remaining crude oil with a driving fluid, e.g., gas, water, brine,steam, polymer solution, foam, or micellar solution. Ideally, suchtechniques (commonly called flooding techniques) provide a bank of oilof substantial depth being driven into a producing well; however, inpractice this is frequently not the case. Oil-bearing strata are usuallyheterogeneous, some parts of them being more permeable than others. As aconsequence, channeling frequently occurs, so that the driving fluidflows preferentially through permeable zones depleted of oil (so-called“thief zones”) rather than through those parts of the strata whichcontain sufficient oil to make oil-recovery operations profitable.

Difficulties in oil recovery due to thief zones may be corrected byinjecting an aqueous solution of an organic polymer and a cross-linkingagent into a subterranean formation under conditions where the polymerwill be cross-linked to produce a gel, thus reducing permeability of thesubterranean formation to the driving fluid (gas, water, etc.).Polysaccharide- or partially hydrolyzed polyacrylamide-based fluidscross-linked with certain aluminum, titanium, zirconium, and boron basedcompounds are used in these enhanced oil recovery applications.Cross-linked fluids or gels, whether for fracturing a subterraneanformation or for reducing permeability of zones in subterraneanformation, are now being used in hotter and deeper wells under a varietyof temperature and pH conditions. In these operations the rate ofcross-linking is critical to the successful generation of viscosity.Frequently the rates of cross-linking with known cross-linkingcompositions are unacceptable, and new, highly specific compositions arerequired.

Commercially available zirconate cross-linkers, such astetra-triethanolamine zirconate cross-link too fast under high pH (pH10) conditions, causing a significant loss in viscosity due to sheardegradation. Other zirconium complexes of triethanolamine, such as thosedisclosed in U.S. Pat. Nos. 4,578,488, 4,683,068, and 4,686,052 can beused as cross-linking agents. However, these complexes also do notcross-link at a desirable rate, especially at high pH conditions,causing a similar loss in viscosity due to shear degradation.

U.S. patent application Ser. No. 11/643,120, filed Dec. 21, 2006,discloses addition of 1 to 20 moles of water per mole of zirconium to atriethanolamine zirconate complex under certain conditions forms astable complex with a 3-8 minute cross-linking rate, while maintainingsatisfactory viscosity development. These cross-linkers have been founddesirable for high temperature operations (149-177° C., 300-350° F.)because of the high initial viscosity they develop, but may be too slowfor low temperature operations (121-149° C., 250-300° F.) and/or may notgenerate sufficient initial viscosity.

U.S. Pat. No. 4,579,670 discloses a general method of controllingreaction rates in a water based polymer fracturing fluid using a mixtureof cross-linker in combination with a cross-linking rate retarder at aratio such that the cross-linking reaction rate is controlled. Thecross-linker employs a transition metal such as titanium, zirconium,chromium or hafnium. Triethanolamine and ethylene glycol are cited asrate retarders.

A glycol may be used as part of a cross-linked fluid or gel, but not aspart of the cross-linker itself. U.S. Pat. Appl. 2006/0027364 disclosesa method of treating subterranean formations using an aqueous gelledfluid comprising an aqueous fluid, a cross-linked guar gelling agent andan amount of a glycol such as ethylene glycol effective to increasestability of the fluid as measured by its viscosity, typically an amountof about 1 to about 10 volume % glycol based on the aqueous fluid. Theglycol is added to the already cross-linked composition. US Pat. Appl.2006/0264334 discloses the use of polyols such as ethylene glycol as asolvent to dissolve polymers used in fracturing fluids, not as part ofthe cross-linker itself.

The need exists for a cross-linker which develops high initial viscosityand which possesses a desirable 3-8 minute rate of cross-linking rateover a broad temperature range (121-177° C., 250-350° F.) for use inhigh pH (about pH 10 and above) fracturing fluids. The present inventionmeets these needs.

SUMMARY OF THE INVENTION

This invention provides a cross-linking composition comprising (a) anaqueous liquid; (b) a pH buffer; (c) a cross-linkable organic polymer;and (d) a solution of a zirconium cross-linking agent comprising theproduct of contacting a zirconium complex with an alkanolamine andethylene glycol wherein the mole ratio of alkanolamine to zirconium is2:1 to 4:1 and the mole ratio of ethylene glycol to zirconium is 1:1 to10:1. Optionally, the zirconium complex may further be contacted withwater, hydroxyalkylated ethylenediamine, or both. If water is added, itis added in an amount to provide a mole ratio of water to zirconium of1:1 to 20:1. Preferably up to 2 moles of hydroxyalkylatedethylenediamine is added per mole of zirconium.

The cross-linking composition of this invention is useful in oil fieldapplications, for example, for hydraulically fracturing a subterraneanformation using the composition. The composition of this invention isfurther useful for plugging permeable zones or leaks in a subterraneanformation. The components of the cross-linking composition may be mixedprior to introducing them into the formation or the components can beintroduced and permitted to react in the formation after a controllableperiod of time.

Surprisingly, in view of known cross-linking compositions comprisingzirconium-triethanolamine complexes, the cross-linking composition ofthis invention has a desirable cross-linking rate of 3-8 minutes andgenerates good viscosity, preferably in the range of 350 to 1000centipoise (Cp) after 90 minutes at pH 10 by simulation in a FANNviscometer at 275° F. (135° C.) and 122 rpm at 100 reciprocal second ofshear.

DETAILED DESCRIPTION OF THE INVENTION

Trademarks and tradenames are shown herein in upper case.

This invention provides an effective cross-linking agent or cross-linkerfor use in cross-linking compositions for oil field applications. Morespecifically, this invention provides a modified zirconium alkanolaminecomplex having an increased rate of cross-linking while providing highinitial viscosity for use in medium to high temperature oil well andhigh pH environment wells. Addition of certain amounts of ethyleneglycol and water to the above zirconium complexes surprisingly increasesthe rate of cross-linking, allowing more rapid, and often higher initialviscosity development. By varying the level of glycol, the increasedrate of cross-linking can be controlled. This discovery is surprisingsince reaction of ethylene glycol with other cross-linkers has beenshown to have no effect or to decrease the rate of cross-linking ratherthan an increase in rate. For example, U.S. Pat. No. 4,579,670 disclosesthe use of ethylene glycol as a delay agent, not an accelerator.

The zirconium complex suitable for cross-linking a fracturing fluid isprepared by a process which comprises contacting a zirconium complexwith an alkanolamine and ethylene glycol and, optionally water, ahydroxyalkylated ethylenediamine, or both, at a mole ratio ofalkanolamine to zirconium of about 2:1 to about 4:1, and a mole ratio ofethylene glycol to zirconium of about 1:1 to about 10:1. The preferredmole ratio of water to zirconium is about 1:1 to about 20:1. Preferablythe amount of zirconium in the complex is between about 1% and 15% byweight. Preferably the amount of ethylene glycol relative to the weightof fracturing fluid is below 0.1% by weight.

The contacting step is preferably performed at a temperature of 50° C.to 90° C. and, for a period of time sufficient to stabilize theresulting zirconium complex.

The process may be divided into steps. For example, a zirconium complexmay be first contacted with alkanolamine to prepare a zirconiumalkanolamine complex, i.e., a solution of triethanolamine zirconate. Thetriethanolamine zirconate solution may be purchased as TYZOR TEAZorganic zirconate, available from E.I. du Pont de Nemours and Company,Wilmington, Del. The zirconium alkanolamine complex may then becontacted with ethylene glycol and optional water and hydroxyalkylatedethylenediamine.

Alternatively, an alkanolamine zirconate solution may be used in theprocess and prepared by contacting a solution of a tetraalkyl zirconatein a C₁-C₆ alcohol with about 2 to about 4 molar equivalents ofalkanolamine per mole of zirconium to produce an initial reactionproduct. Preferably the molar ratio of alkanolamine to zirconium isabout 4.

A number of tetraalkyl zirconates (also known as zirconiumtetraalkoxides) can be used to prepare the zirconium cross-linking agentin the composition of this invention, e.g., tetra-1-propyl zirconate,tetra-n-propyl zirconate, and tetra-n-butyl zirconate. The preferredtetraalkyl zirconate is tetra-n-propyl zirconate, available as TYZOR NPZorganic zirconate, a solution in n-propanol, with a zirconium content asZrO₂ of about 28% by weight, available from E.I. du Pont de Nemours andCompany, Wilmington, Del.

The alkanolamine may be selected from the group consisting oftriethanolamine, triisopropanolamine, and tri-n-propanolamine. Thepreferred alkanolamine is triethanolamine (TEA).

When the zirconium cross-linking agent contains a hydroxyalkylatedethylenediamine, the agent may be prepared by adding a hydroxyalkylatedethylenediamine to an alkanolamine zirconate solution. When ahydroxyalkylated ethylenediamine is used, it is added in an amount up toabout 2 molar equivalents, per mole of zirconium. Preferably thehydroxyalkylated ethylene diamine isN,N,N′,N′-tetrakis-(2-hydroxypropyl)ethylenediamine, availablecommercially, for example, from BASF Corporation, Mount Olive, N.J.,under the name QUADROL polyol.

When a zirconium alkanolamine complex or zirconiumalkanolamine-hydroxyalkylated ethylenediamine complex is prepared, thecomplex is modified by adding about 1 to about 10 moles of ethyleneglycol per mole of zirconium and optionally a small amount of water, andmixing and reacting until a new equilibrium is established. The reactionmay be carried out at a temperature of 50° C. to 90° C. A contact timeof about 4 hours at 80° C. is adequate, but other periods may be used.

Alternatively, ethylene glycol, zirconium complex, and alkanolamine andoptionally hydroxyalkylated ethylenediamine, water, or both are added inany order and reacted. If water is added, it is preferably added afterthe other ingredients have been mixed to avoid possible precipitation ofzirconium hydroxides by reaction with tetraalkyl zirconate.

The cross-linking composition of this invention comprises (a) an aqueousliquid; (b) a pH buffer; (c) a cross-linkable organic polymer; and (d) azirconium cross-linking agent prepared by a process comprisingcontacting a zirconium complex with an alkanolamine and ethylene glycol,and optionally one or both of a hydroxyalkylated ethylenediamine andwater, wherein the agent has a mole ratio of alkanolamine to zirconiumof 2:1 to 4:1, a mole ratio of 1:1 to 10:1 ethylene glycol to zirconium,up to a mole ratio of 2:1 hydroxyalkylated ethylenediamine to zirconium,and optionally a mole ratio of 1:1 to 20:1 water to zirconium.

The aqueous liquid (a) is typically selected from the group consistingof water, aqueous alcohol, and aqueous solution of a clay stabilizer.The alcohol can be the same or different alcohol as the reactionsolvent, that is, an alcohol having 1 to 6 carbon atoms. Preferably,when the aqueous liquid is aqueous alcohol, the alcohol is methanol orethanol. Clay stabilizers include, for example, hydrochloric acid andchloride salts, such as, tetramethylammonium chloride (TMAC) orpotassium chloride. Aqueous solutions comprising clay stabilizers maycomprise, for example, 0.05 to 0.5 weight % of the stabilizer, based onthe combined weight of the aqueous liquid and the organic polymer (i.e.,the base gel). Preferably, when the aqueous liquid is an aqueoussolution of a clay stabilizer, the clay stabilizer istetramethylammonium chloride or potassium chloride.

The aqueous liquid can also be a mixture of water and one or moreorganic solvents. Organic solvents that may be used include alcohols,glycols, polyols, and hydrocarbons such as diesel.

Preferably, the aqueous liquid is water, aqueous methanol, aqueousethanol, an aqueous solution of potassium chloride, an aqueous solutionof tetramethylammonium chloride, or a combination of two or morethereof.

The cross-linking composition comprises an effective amount of a pHbuffer (b) to control pH. The pH buffer may be acidic, neutral or basic.The pH buffer is generally capable of controlling the pH from about pH 5to about pH 12. For example in a composition for use at a pH of 5-7, afumaric acid-based buffer or a sodium diacetate-based buffer can beused. In a composition for use at a pH of 7-8.5, a sodiumbicarbonate-based buffer can be used. In a composition for use at a pHof 9-12, a sodium carbonate or sodium hydroxide-based buffer can beused. The cross-linking composition solution of this inventionadvantageously comprises a pH buffer for pH 9-12. Other suitable pHbuffers can be used, as are known to those skilled in the art.

The composition further comprises a cross-linkable organic polymer (c).Suitable cross-linkable organic polymers are selected from the groupconsisting of solvatable polysaccharides, polyacrylamides andpolymethacrylamides. Preferably the organic polymer is a solvatablepolysaccharide and is selected from the group consisting of gums, gumderivatives and cellulose derivatives. Gums include guar gum and locustbean gum, as well as other galactomannan and glucomannan gums, such asthose derived from sennas, Brazilwood, tera, honey locust, karaya gumand the like. Preferred gum derivatives include hydroxyethylguar (HEG),hydroxypropylguar (HPG), carboxyethylhydroxyethylguar (CEHEG),carboxymethylhydroxypropylguar (CMHPG), and carboxymethyl guar (CMG).Preferred cellulose derivatives include those containing carboxylgroups, such as carboxymethylcellulose (CMC) andcarboxymethylhydroxyethylcellulose (CMHEC). The solvatablepolysaccharides can be used individually or in combination; usually,however, a single material is used. Guar derivatives and cellulosederivatives are preferred, such as, HPG, CMC and CMHPG. HPG is generallymore preferred based upon its commercial availability and desirableproperties. However, CMC and CMHPG may be more preferred incross-linking compositions when the pH of the composition is less than6.0 or higher than 9.0, or when the permeability of the formation issuch that one wishes to keep the residual solids at a low level toprevent damage to the formation. The cross-linkable polymer is normallymixed with the aqueous liquid to form a base gel.

The zirconium cross-linking agent (d) is prepared by contacting azirconium complex with an alkanolamine and ethylene glycol, wherein themole ratio of alkanolamine to zirconium is 2:1 to 4:1, and the moleratio of ethylene glycol to zirconium is 1:1 to 10:1. Optionally, thezirconium complex is further contacted with water, hydroxyalkylatedethylenediamine, or both, as described previously.

The cross-linking composition may comprise optional components,including those which are common additives for oil field applications.Thus, the composition may further comprise one or more of proppants,friction reducers, bactericides, hydrocarbons, chemical breakers,polymer stabilizers, surfactants, formation control agents, and thelike. Proppants include sand, bauxite, glass beads, nylon pellets,aluminum pellets and similar materials. Friction reducers includepolyacrylamides. Hydrocarbons include diesel oil. Chemical breakersbreak the cross-linked polymer (gel) in a controlled manner and includeenzymes, alkali metal persulfate, and ammonium persulfate. Polymerstabilizers include methanol, alkali metal thiosulfate, and ammoniumthiosulfate.

These optional components are added in an effective amount sufficient toachieve the desired cross-linking performance based on the individualcomponents, desired cross-linking time, temperature and other conditionspresent in the formation being fractured or permeable zone beingplugged.

The cross-linking composition is produced by mixing the zirconiumcross-linking agent with the other components, in any order. Forexample, in one particular application in an oil field, the zirconiumcross-linking agent and optional components are introduced into aformation, while the cross-linkable organic polymer and aqueous liquidare introduced into the formation as a separate stream. Alternatively,all components may be premixed and introduced into a subterraneanformation as a single stream. Advantageously, the components may bemixed in different combinations, and more advantageously, the componentsmay be mixed just prior to use to enable easy variation and adjustmentof the cross-linking rate.

This invention provides a method for hydraulically fracturing asubterranean formation, which comprises introducing into the formationat a flow rate and pressure sufficient to create, reopen, and/or extendone or more fractures in the formation, a cross-linking compositioncomprising: (a) an aqueous liquid; (b) a pH buffer; (c) a cross-linkableorganic polymer; and (d) a zirconium cross-linking agent prepared by aprocess comprising contacting a zirconium complex with an alkanolamineand ethylene glycol, and optionally one or both of a hydroxyalkylatedethylenediamine and water, wherein the agent has a mole ratio ofalkanolamine to zirconium of 2:1 to 4:1, a mole ratio of 1:1 to 10:1ethylene glycol to zirconium, up to a mole ratio of 2:1 hydroxyalkylatedethylenediamine to zirconium, and optionally a mole ratio of 1:1 to 20:1water to zirconium. Preferably, the alkanolamine is triethanolamine.

In one embodiment of the method for hydraulically fracturing asubterranean formation, the zirconium cross-linking agent and a base gelare contacted prior to their introduction into the formation, such thatthe cross-linking agent and polymer react to form a cross-linked gel.The cross-linked gel is then introduced into the formation at a flowrate and pressure sufficient to create, reopen, and/or extend a fracturein the formation. In this method, a base gel is prepared by mixing across-linkable organic polymer with an aqueous liquid. The cross-linkedgel is prepared by mixing the base gel with a zirconium cross-linkingagent prepared by a process comprising contacting a zirconium complexwith an alkanolamine and ethylene glycol, and optionally one or both ofa hydroxyalkylated ethylenediamine and water, wherein the agent has amole ratio of alkanolamine to zirconium of 2:1 to 4:1, a mole ratio of1:1 to 10:1 ethylene glycol to zirconium, up to a mole ratio of 2:1hydroxyalkylated ethylenediamine to zirconium, and optionally a moleratio of 1:1 to 20:1 water to zirconium. At least one of the zirconiumcross-linking agent and the base gel further comprise a pH buffer.

Alternatively, the subterranean formation may be penetrated by awellbore, such that contacting the solution of zirconium cross-linkingagent with the base gel occurs in the wellbore and the cross-linked gelis introduced into the formation from the wellbore. This method ofhydraulically fracturing a subterranean formation penetrated by awellbore comprises (a) preparing a base gel by mixing a cross-linkableorganic polymer with an aqueous liquid; (b) introducing the base gelinto the wellbore; (c) simultaneously with, or sequentially after,introducing the base gel into the wellbore, introducing a zirconiumcross-linking agent prepared by a process comprising contacting azirconium complex with an alkanolamine and ethylene glycol, andoptionally one or both of a hydroxyalkylated ethylenediamine and water,wherein the agent has a mole ratio of alkanolamine to zirconium of 2:1to 4:1, a mole ratio of 1:1 to 10:1 ethylene glycol to zirconium, up toa mole ratio of 2:1 hydroxyalkylated ethylenediamine to zirconium, andoptionally a mole ratio of 1:1 to 20:1 water to zirconium; (d)permitting the base gel and the solution of zirconium cross-linkingagent to react to form a cross-linked aqueous gel; and (e) introducingthe cross-linked gel into the formation from the wellbore at a flow rateand pressure sufficient to create, reopen, and/or extend a fracture inthe formation. A pH buffer is independently admixed with the base gel,the zirconium cross-linking agent or both prior to introducing the basegel and the zirconium cross-linking agent into the wellbore.

Upon creation of a fracture or fractures, the method may furthercomprise introducing a cross-linking composition comprising the solutionof zirconium complex, a cross-linkable organic polymer and proppant intothe fracture or fractures. This second introduction of a solution ofzirconium cross-linking agent is preferably performed in the event thecross-linking composition used to create the fracture or fractures didnot comprise proppant.

Another use for the zirconium cross-linking agent of the presentinvention relates to a method for selectively plugging permeable zonesand leaks in subterranean formations which comprises introducing intothe permeable zone or the site of the subterranean leak, a cross-linkingcomposition comprising (a) an aqueous liquid; (b) a pH buffer; (c) across-linkable organic polymer; and (d) a zirconium cross-linking agentprepared by a process comprising contacting a zirconium complex with analkanolamine and ethylene glycol, and optionally one or both of ahydroxyalkylated ethylenediamine and water, wherein the agent has a moleratio of alkanolamine to zirconium of 2:1 to 4:1, a mole ratio of 1:1 to10:1 ethylene glycol to zirconium, up to a mole ratio of 2:1hydroxyalkylated ethylenediamine to zirconium, and optionally a moleratio of 1:1 to 20:1 water to zirconium; into the permeable zone or thesite of the subterranean leak. The pH buffer may be admixed with thezirconium cross-linking agent prior to introducing the cross-linkingcomposition into the permeable zone or site of the leak.

In a first embodiment of the method for plugging a permeable zone or aleak in a subterranean formation, the aqueous liquid, pH buffer,cross-linkable organic polymer and the zirconium cross-linking agent arecontacted prior to their introduction into the subterranean formation,such that the polymer and cross-linking agent react to form across-linked aqueous gel, which gel is then introduced into theformation.

In an alternative embodiment of the method for plugging a permeable zoneor a leak in a subterranean formation, the zirconium cross-linking agentand the cross-linkable organic polymer are introduced separately, eithersimultaneously or sequentially, into the permeable zone or the site ofthe subterranean leak such that cross-linking occurs within thesubterranean formation. This method comprises (a) preparing a base gelby mixing a cross-linkable organic polymer with an aqueous liquid; (b)introducing the base gel into the into the permeable zone or the site ofthe subterranean leak, (d) simultaneously with, or sequentially after,introducing the base gel into the into the permeable zone or the site ofthe subterranean leak, introducing the zirconium cross-linking agentinto the into the permeable zone or the site of the subterranean leak;(e) permitting the base gel and the cross-linking agent to react to forma cross-linked aqueous gel to plug the zone and/or leak. The zirconiumcross-linking agent, the base gel, or both further comprise a pH buffer.

The relative amounts of cross-linkable organic polymer and the zirconiumcross-linking agent may vary. One uses small but effective amounts whichfor both will vary with the conditions, e.g., the type of subterraneanformation, the depth at which the method (e.g., fluid fracturing,permeable zone plugging or leak plugging) is to be performed,temperature, pH, etc. Generally one uses as small an amount of eachcomponent as will provide the viscosity level necessary to effect thedesired result, i.e., fracturing of the subterranean formation, orplugging permeable zones or leaks to the extent necessary to promoteadequate recovery of oil or gas from the formation.

For example, satisfactory gels can generally be made for fluidfracturing by using the cross-linkable organic polymer in amounts up toabout 1.2 weight % and the cross-linking composition in amounts up toabout 0.50 weight % of the zirconium cross-linking agent, withpercentages being based on the total weight of the base gel. Preferably,from about 0.25 to about 0.75 weight % of the cross-linkable organicpolymer is used and from about 0.05 to about 0.25 weight % of thezirconium complex is used.

In a method for plugging permeable zones or leaks, generally about 0.25to 1.2 weight % of a cross-linkable organic polymer is used, preferably0.40 to 0.75 weight %, based on the total weight of the base gel.Generally about 0.01 to 0.50 weight % of the zirconium cross-linkingagent is used, preferably 0.05 to 0.25 weight %, based on the totalweight.

The amount of zirconium cross-linking agent used to cross-link theorganic polymer is that which provides a zirconium ion concentration ina range from about 0.0005 weight % to about 0.1 weight %, based on thetotal weight of the base gel. The preferred concentration of zirconiumion is in the range of from about 0.001-0.05 weight %, based on thetotal weight.

Typically the zirconium cross-linking agent of this invention can beused at a pH of from about 3 to 11. For low temperature applications(150-250° F., 66-121° C.), carbon dioxide-based energized fluids may beused. In this case, a pH for the cross-linking composition of about 3 toabout 6 is preferred. For moderate or high temperature applications(275-400° F., 121-204° C.), a pH of about 9 to about 11 is preferred.Advantageously, the solution of zirconium complex of this invention isused at a temperature of 275-325° F. (135-163° C.) and at a pH 10 orgreater. For successful completion of the fracturing operation at atemperature of 250° F. (121° C.), whether hydraulic fracturing orplugging a permeable zone, the cross-linking composition should providean initial viscosity of at least 650 Cp, preferably at least 750 Cp, anda viscosity of at least 350 Cp, preferably at least 500 Cp, 90 minutesafter introducing the cross-linking composition into the subterraneanformation or permeable zone or site of a subterranean leak.

EXAMPLES

The preparation of the compositions in the Examples and in theComparative Examples were each carried out in closed vessels containingan agitator, thermometer, condenser, nitrogen inlet and dropping funnel.Unless specified otherwise, percentages are given by weight.Temperatures are given in degrees Celsius. The cross-linking propertiesof the compositions of this invention are given in the Examples as afunction of the viscosity of carboxymethylhydroxypropylguar cross-linkedwith the zirconate of this invention.

Preparation of Base Gel

A Waring blender jar was filled with 1 liter of distilled water. To thiswas added 2 g of a 50% aqueous solution of tetramethylammonium chlorideclay stabilizer. Agitation was started and 3.6 g ofcarboxymethylhydroxypropylguar (CMHPG) was sprinkled into the vortex ofthe agitating solution. The pH of the resultant slurry was adjusted to 6with sodium diacetate and agitation continued for 30 minutes. The pH wasthen adjusted to 10.3 with 10% sodium hydroxide solution. Agitation wasstopped and the gel was allowed to stand for 30 minutes or more beforeuse.

Viscosity Measurement of Zirconate Cross-Linked Base Gel

To 250 ml of a vigorously agitated sample of base gel in a Waringblender jar, was added 0.00032 moles of zirconium (0.2-1.0 ml dependenton percent zirconium of cross-linker solution—hereinafter referred to asthe Standard Loading Density). Agitation was continued for about 15-180seconds. A 25-ml sample of the cross-linker containing gel was placed inthe cup of the FANN 50 Viscometer with an R-1, B-3 configuration andviscosity was measured at 275° F. (135° C.) and 122 rpm at 100reciprocal seconds of shear.

Comparative Example A

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 59.8 g of 85% lactic acid. Agitation wasstarted and 61.6 g of triethanolamine and 50 g of water were added. Thesolution was cooled to 15° C. and then 122.5 g of 30% zirconiumoxychloride solution were added. The pH was adjusted to 8.0 using 21.7 gof 28% ammonium hydroxide solution. The solution was diluted with 180 gof water to give 495 g of a water white solution containing 3.8% of Zr.

Comparative Example B

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and a mixture of 67.6 g of triethanolamine and 21g of water was added. The solution was heated at 80° C. for 4 hours togive 188.6 g of an orange liquid containing 11% of Zr.

Comparative Example C

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 180 g of TYZOR TEAZ organic zirconate.Agitation was started and 20 g of water were added. The solution wasagitated for 4 hours at 80° C. to give 200 g of an orange solutioncontaining 11.9% Zr.

Comparative Example D

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 313.7 g of TYZOR TEAZ organic zirconate.Agitation was started and 132.6 g of QUADROL polyol were added. Thesolution was agitated for 2 hours at 60° C. to give 445 g of an orangesolution containing 9.3% Zr.

Comparative Example E

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and 135.2 g of triethanolamine were added. Thesolution was heated to 60° C. and held at this temperature for 2 hours.Then, 66.2 g of QUADROL polyol were added and the solution was agitatedfor an additional 2 hours at 60° C. Finally, 10.5 g of water were addedand the solution was held at 60° C. for yet another 2 hours to give 304g of an orange solution containing 6.6% Zr.

Comparative Example F

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and 135.2 g of triethanolamine were added. Thesolution was heated to 60° C. and held at this temperature for 2 hours.Then, 66.2 g of QUADROL polyol were added and the solution agitated foran additional 2 hours at 60° C. Finally, 21 g of water was added and thesolution was held at 60° C. for yet another 2 hours to give 314 g of anorange solution containing 6.4% Zr.

Example 1

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and a mixture of 67.6 g of triethanolamine, 14.1 gof ethylene glycol and 21 g of water was added. The solution was heatedat 80° C. for 4 hours to give 202 g of an orange liquid containing 10.2%of Zr.

Example 2

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and a mixture of 67.6 g of triethanolamine, 56.3 gof ethylene glycol and 21 g of water was added. The solution was heatedat 80° C. for 4 hours to give 244 g of an orange liquid containing 8.5%of Zr.

Example 3

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and a mixture of 135.2 g of triethanolamine, 14.1g of ethylene glycol and 21 g of water was added. The solution washeated at 80° C. for 4 hours to give 270 g of an orange liquidcontaining 7.7% of Zr.

Example 4

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and a mixture of 135.2 g of triethanolamine, 56.3g of ethylene glycol and 21 g of water was added. The solution washeated at 80° C. for 4 hours to give 293 g of an orange liquidcontaining 6.6% of Zr.

Example 5

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 156.8 g of TYZOR TEAZ organic zirconate.Agitation was started and a mixture of 66.2 g of QUADROL polyol and 56.3g of ethylene glycol was added. The solution was agitated for 4 hours at80° C. to give 279 g of an orange solution containing 7.4% Zr.

Example 6

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and 135.2 g of triethanolamine were added. Thesolution was heated to 60° C. and held at this temperature for 2 hours.Then, 66.2 g of QUADROL polyol were added and the solution was agitatedfor an additional 2 hours at 60° C. Finally, a mixture of 56.2 g ofethylene glycol and 10.5 g of water was added and the solution was heldat 60° C. for yet another 2 hours to give 361 g of an orange solutioncontaining 5.6% Zr.

Example 7

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 100 g of TYZOR NPZ organic zirconate.Agitation was started and 135.2 g of triethanolamine were added. Thesolution was heated to 60° C. and held at this temperature for 2 hours.Then, 66.2 g of QUADROL polyol were added and the solution was agitatedfor an additional 2 hours at 60° C. Finally, a mixture of 56.2 g ofethylene glycol and 21 g of water was added and the solution was held at60° C. for yet another 2 hours to give 371 g of an orange solutioncontaining 5.5% Zr.

Table 1 shows the performance of a CMHPG gel cross-linked with zirconiumtriethanolamine complexes (Comparative Examples A and B, with varyingamounts of ethylene glycol added to Comparative Example B). In thisTable, “% Zr” refers to percent of zirconium in the solution of thezirconium cross-linking agent; “Zr, mL” refers to the milliliters ofcross-linker solution injected in the test. “TEA” is triethanolamine;“LA” is lactic acid. Notes 1 and 2 refer to additional steps taken forseparate runs using Comparative Example B.

“Fann Time at Max Viscosity, min.” means the time in minutes it takes toreach maximum viscosity. The viscosity at this maximum time is labeled“Max Viscosity, Cp” to indicate viscosity in centipoise (Cp). Theviscosity after 90 minutes at the test temperature of 275° F. (135° C.)is labeled “Viscosity, Cp, at 90 min.”

Table 2 uses the same column headings as Table 1, and adds thefollowing: “QUADROL” is tetrahydroxyisopropyl ethylenediamine.

TABLE 1 Fann Alpha- Time at Ethylene hydroxy Max Max. TEA, glycol, acidWater, Viscosity, Viscosity, Viscosity, Example % Zr Zr, ml Zr, molesmoles moles (moles) moles min. Cp Cp, at 90 min. Comp. A 3.8 0.78 1 2 LA(3) 6.5 490 75 Comp. B 11 0.27 1 2 5.12 6 545 230 Comp. B (Note 1) 112.5 1 2 134 5.12 7.5 490 325 Comp. B (Note 2) 11 25 1 2 1340 5.12 10 14596 (Note 1): The composition from Comparative Example B was mixed with1% by weight of ethylene glycol. (Note 2): The composition fromComparative Example B was mixed with 10% by weight of ethylene glycol.

As can be seen from results in Table 1, an aqueous zirconatecross-linker prepared according to Comparative Example A, as disclosedin U.S. Pat. No. 4,524,829 or 4,460,751, cross-links within the desired3-8 minute time frame, however, has low initial viscosity and does notretain adequate viscosity (>100 Cp) at 135° C., 275° F., for the desired90 minutes.

It can be further seen from Table 1, addition of ethylene glycol inamounts of 1% and 10% (134 and 1340 moles per mole of zirconium), delaysthe rate of cross-linking of zirconium triethanolamine complexes(Comparative Example B) and can dramatically decrease viscositygeneration and destabilize viscosity retention. When 1% ethylene glycolis added to Comparative Example B cross-linking rate slows or isdecreased (7.5 minutes vs. 5 minutes). When 10% ethylene glycol isadded, not only does cross-linking rate further decreased (10 minutesvs. 5 minutes), but the viscosity generation capability and retention issignificantly reduced (96 Cp vs. 230 Cp after 90 minutes). Thus, highlevels of ethylene glycol are deleterious to the cross-linkingcompositions of this invention.

Table 2 shows the performance of a CMHPG gel cross-linked with zirconiumtriethanolamine complexes of the invention in Examples 1-7 andComparative Examples B-F. As can be seen from Table 2, lower levels ofethylene glycol (1 to 4 moles per mole of zirconium) accelerate the rateof cross-linking of zirconium triethanolamine complexes, as shown byviscosity generation and retention is stabilized (increased). As shownby Examples 5, 6, and 7, addition of water retards the rate ofcross-linking of zirconium triethanolamine complexes in the presence ofethylene glycol. Examples 1-7 show a faster rate of cross-linking thantheir respective Comparison Examples B-F, and generate higherviscosities, which ensure adequate viscosity retention during the courseof the fracturing operation.

TABLE 2 Fann Time Ethylene at Max Max. Zr, TEA, QUADROL, Glycol,Viscosity, Viscosity, Viscosity, Example % Zr Zr, ml moles moles molesmoles Water min. Cp Cp, at 90 min. Comp. B 11 0.27 1 2 5.12 6 545 230 110.2 0.29 1 2 1 5.12 4.5 1275 690 2 8.5 0.35 1 2 4 5.12 4 1300 760 Comp.C 11.9 0.2 1 4 5.12 8 690 650 3 7.7 0.39 1 4 1 5.12 4.5 1200 725 4 6.60.45 1 4 4 5.12 3.5 1675 1050 Comp. D 9.3 0.32 1 4 1 0 4.5 900 550 5 7.40.4 1 4 1 4 0 4 1210 770 Comp. E 6.6 0.45 1 4 1 2.56 12 512 404 6 5.60.53 1 4 1 4 2.56 7 724 455 Comp. F 6.4 0.46 1 4 1 5.12 13 519 321 7 5.50.54 1 4 1 4 5.12 8 696 395

What is claimed is:
 1. A cross-linking composition comprising (a) an aqueous liquid; (b) a pH buffer; (c) a cross-linkable organic polymer; and (d) a zirconium cross-linking agent comprising the product of contacting a solution of a zirconium complex with an alkanolamine and ethylene glycol as an accelerator wherein the mole ratio of alkanolamine to zirconium is 2:1 to 4:1 and the mole ratio of ethylene glycol to zirconium is 1:1 to 10:1 wherein the cross-linking composition has a crosslinking rate in the range of 3-8 minutes, providing an initial viscosity of at least 650 Cp, and a viscosity of at least 350 Cp after 90 minutes, further wherein, if the zirconium complex is contacted with water, water acts as a rate retarder, and water is added in an amount to provide a mole ratio of water to zirconium of 1:1 to 20:1.
 2. The composition of claim 1 wherein the zirconium complex is further contacted with water at a mole ratio of water to zirconium of 1:1 to 20:1.
 3. The composition of claim 1 or 2 wherein the zirconium complex is further contacted with a hydroxyalkylated ethylenediamine at a mole ratio of greater than 0 up to 2:1 of hydroxyalkylated ethylenediamine to zirconium.
 4. The composition of claim 3 wherein the hydroxyalkylated ethylenediamine is N,N,N′,N′-tetrakis-(2-hydroxypropyl)ethylenediamine.
 5. The composition of claim 1 wherein the alkanolamine is selected from the group consisting of triethanolamine, triisopropanolamine, and tri-n-propanolamine.
 6. The composition of claim 5 wherein the alkanolamine is triethanolamine.
 7. The composition of claim 1 wherein the zirconium complex is tetra-1-propyl zirconate, tetra-n-propyl zirconate, or tetra-n-butyl zirconate.
 8. The composition of claim 7 wherein the zirconium complex is tetra-n-propyl zirconate.
 9. The composition of claim 1 wherein the cross-linking composition provides a viscosity in the range of 350 to 1000 Cp after 90 minutes.
 10. The composition of claim 1 wherein the cross-linking composition provides an initial viscosity of at least 750 Cp, and a viscosity of at least 500 Cp after 90 minutes. 